If you aren't too concerned about space, and only need speed, then I suggest dumping lagarith and MJPEG and use UTVideo directly:
http://umezawa.dyndns.info/wordpress/?p=1770Comes both in x86 and x64 flavour. It also has proper multi-core support. Remember to properly set the threads you want to give it, there's a check for the automatic thread detection too, if you wish.
This one has an amazing speed for decidedly acceptable compression ratios. However, lagarith still has the upper hand in compression, albeit slightly. UTVideo still manages better compression than HuffYUV, though, and it IS way faster.
Here's a lossless codec comparison:
http://saber.kawaii-shoujo.net/Various/lossless.temp.v2-nox264.txtAs you can see, if you optimise it for compression, you still get about 400fps decoding speed, whereas decoding optimisation gets you over 500fps decoding speed (well, with that footage and on that system, take those speeds as %).
This should make editing fast enough without the need of proxy clips even with HD footage.